1. Field of the Invention
The present invention relates to fault tolerant data transmission and, more particularly, to a fiber optic fault tolerant protocol acquiring synchronization and recognizing the beginning of data and error correction group boundaries despite the occurrence of burst errors in the received stream of characters.
2. Statement of the Problem
In fiber optic data transmission systems (and other types of data transmission systems), "burst" errors occur. "Burst" errors are usually four to five bits in length, are based on a Gaussian distribution, and as the rate of data transmission increases, are more likely to occur.
A number of conventional Error Correction (EC) techniques are available for correcting errors in frames of the transmitted data. A problem exists with conventional EC techniques when burst errors occur in the framing information used to indicate the beginning of the data (BOD), which corresponds with the beginning of a plurality of Error Correction Groups (ECGs). When this class of errors (i.e., framing information errors) occurs, it is not possible to correct the subsequent ECGs since the location of the BOD is unknown. When framing information burst errors exist, EC techniques are not applicable and the frame must be retransmitted or lost. Hence, a need exists to provide the location of the BOD and/or ECG boundaries even in the presence of multiple burst errors occurring in the framing information. The framing information contains a plurality of identical sync characters that provide synchronization by the receiving circuitry with the transmitted frame. When burst errors occur in the sync characters, synchronization may not occur and the frame must be retransmitted or lost. A need also exists to acquire synchronization for the frame even in the presence of multiple burst errors. Conventionally, different types of framing data may follow the framing information. A need also exists to provide information as to the identity of the ECGs following the BOD. When synchronization is not acquired or when the BOD is not detected, a need exists to provide an overall error flag.
3. Prior Approaches
The 1971 patent to Tong (U.S. Pat. No. 3,571,794) pertains to an automatic synchronization recovery for data systems utilizing burst error-correcting cyclic codes. This invention handles errors occurring simultaneously at each end but not in the middle of a data word. A specific preselected data sequence is added to each data word transmitted and the same fixed sequence is subtracted from the received data sequences. This approach distinguishes itself from the aforesaid statement of the problem by providing separate information along with the data.
The 1991 patent to Sakai et al. (U.S. Pat. No. 5,038,351) pertains to a coded mark inversion (CMI) block sync circuit. This approach is based on the CMI code wherein a binary code "1" is represented by one of alternately repeated "00" or "11" and wherein "0" of a binary code is represented by either "01" or "10." Sakai counts violated bits of input CMI codes within a supervisory section. This approach distinguishes itself from the statement of the problem through utilization of the CMI code.
The 1979 patent to Bench et al. (U.S. Pat. No. 4,156,867) uses a start code to define the beginning of the data block and to enable synchronization. The start code has a set number of a correlatable pattern of binary bits such as thirty-two.
4. Solution of the Problem
The present invention provides solutions to the above-stated four problems by employing two burst sequences in the framing information that together allow for a maximum statistical recognition of the BOD and/or the ECG boundaries. The present invention utilizes the technique herein defined as "sync burst." "Sync burst" is a burst of multiple sync characters that allows for the detection of a predetermined sequential number of sync characters in the presence of multiple channel burst errors. The present invention teaches that by requiring multiple sync characters, the possibility of false synchronization is significantly reduced. Upon detection of the predetermined sequential number of sync characters, synchronization is acquired and a sync signal is issued.
The second feature of the present invention is the provision of a preamble burst immediately following the sync burst. The preamble burst as used herein is designed as a known sequence of characters in which some majority of these characters are correctly recognized so as to provide the BOD. By incorporating both sync burst and preamble burst, the BOD is recognized even when a certain number of channel burst errors are present in the framing information.
Associated with each character in the preamble burst is a nibble identifying the type of upcoming ECG. This third feature is optional with the present invention.
Finally, if detection of the preamble burst does not occur within a predetermined time interval after the sync signal is issued, then an overall error flag is raised.